The somatosensory system can detect changes in ambient temperature over a remarkably wide range, enabling us to discriminate among thermal stimuli of an innocuous (cool or warm) or noxious (cold or heat) quality. This process is initiated when a thermal stimulus excites the peripheral terminals of primary sensory neurons from dorsal root or trigeminal ganglia, which innervate regions of the trunk and head, respectively. These neurons convert thermal stimuli into electrochemical signals (i.e. action potentials) and relay information to integrative centers in the spinal cord and brain (Fields, Pain (1987); Julius & Basbaum, Nature 413:203-10 (2001)). Noxious (painful) heat is detected by primary sensory neurons that respond with a moderate thermal threshold of ˜43° C. or with a high threshold of ˜52° C. (Raja et al., in Textbook of Pain, pages 11-57 (Wall & Melzack, eds., 1999); Nagy & Rang, Neuroscience 88:995-7 (1999)). Insights into the molecular mechanisms of heat sensation have come from the cloning and characterization of the vanilloid receptor (VR1), an excitatory ion channel on sensory neurons that is activated by capsaicin, the main pungent ingredient in “hot” chili peppers and is also activated by noxious heat at temperatures >43° C. (Caterina et al., Nature 389:816-24 (1997)). Indeed, electrophysiological, anatomical, and genetic studies support this hypothesis and show that VR1 is essential for the development of thermal hypersensitivity following tissue injury (Caterina & Julius, Annu. Rev. Neurosci. 24:487-517 (2001)). A related ion channel, VRL-1, does not respond to capsaicin, but is activated by temperatures >50° C., suggesting that it contributes to heat sensitivity of high threshold neurons (Caterina et al., Nature 398:436-41 (1999)). Both VR1 and VRL-1 belong to the transient receptor potential (TRP) ion channel family.
In contrast to the understanding of noxious heat sensation, little is known about how we detect cold. Recordings from cutaneous sensory nerves in the cat suggest that noxious cold (<15° C.) is detected primarily by two classes of unmyelinated C-fibers: those that also respond to high-threshold (noxious) mechanical and heat stimuli, and another population that is activated by low-threshold (innocuous) mechanical stimuli.
Another class of C-fibers can be activated by moderate cooling of the skin to 25° C., but are mechanically insensitive (Bessou & Perl, J. Neurophysiol. 32:1025-43 (1969)). Interestingly, some fibers in this latter class are also activated at temperatures >43° C., a phenomenon classically described as a paradoxical response of cold fibers to noxious heat (Campero et al., J. Physiol. 535:855-65 (2001); Dodt & Zotterman, Acta Physiol. Scand. 26:358-365 (1952)). Studies in rodents show that unmyelinated C-fibers as well as thinly myelinated Aδ fibers are sensitive to noxious cold, but the percentage of such units responding to cold ranges from ˜10% to 100%, depending on the stimulus intensity and species examined (Kress et al., J. Neurophysiol. 68:581-95 (1992); Caterina et al., Science 288:306-13 (2000); Simone & Kajander, Neurosci. Lett. 213:53-6 (1996); Simone & Kajander, J. Neurophysiol. 77:2049-60 (1997); Cain et al., J. Neurophysiol. 85:1561-74 (2001)).
This wide variability in the literature may reflect the fact that thermal thresholds for cold-sensitive fibers are not as well defined as they are for heat-sensitive units. Moreover, psychophysical thresholds for cold-evoked pain are not as precise as they are for heat and thus fiber types that transduce sensations of innocuous cool or noxious cold are not as firmly established. At the cellular level, calcium-imaging and patch-clamp studies of dissociated dorsal root ganglion (DRG) neurons have shown that cold (˜20° C.) promotes calcium influx, possibly through the direct opening of calcium-permeable ion channels on these cells (Reid & Flonta, Nature 413:480 (2001); Suto & Gotoh, Neuroscience 92:1131-5 (1999)). However, several other mechanisms have been proposed to explain cold-evoked membrane depolarization, including inhibition of background K+ channels (Reid & Flonta, Neurosci. Lett. 297:171-4 (20010), activation of Na+-selective degenerin channels (Askwith et al., Proc. Natl. Acad. Sci. U.S.A. 98:6459-63 (2001)), inhibition of (Na+/K+) ATPases (Pierau et al., Brain Res. 73:156-60 (1974)), or differential effects of cold on voltage-gated Na+ and K+ conductances (Braun et al., Pflugers Arch. 386:1-9 (1980)). Thus it is not clear whether cold excites sensory neurons by activating a discrete “cold receptor,” or by modulating a constellation of excitatory and inhibitory channels on these cells.
Fifty years ago, Hensel & Zotterman (Acta Physiol. Scand. 24:27-34 (1951)) showed that menthol potentiates responses of trigeminal fibers to cold by shifting their thermal activation thresholds to warmer temperatures. Moreover, they proposed that cooling compounds mediate their psychophysical effects by interacting with a protein in sensory neurons that is specific to the process of cold transduction. Although recent studies of sensory nerve fibers or dissociated DRG neurons in culture support this idea, no unifying cellular mechanism has been proposed to explain menthol's action. For example, one model proposes that menthol inhibits voltage-dependent Ca2+ channels (Swandulla et al., Pflugers Arch. 409:52-9 (1987)), thereby decreasing activation of Ca2+-dependent K+ channels and prolonging depolarization of cold-sensitive fibers (Schafer et al., J Gen Physiol. 88:757-76 (1986)). Another model predicts that menthol directly activates calcium-permeable ion channels on these cells (Reid & Flonta, Nature 413:480 (2001); Okazawa et al., Neuroreport 11: 2151-5 (2000)). In any case, there is currently no direct pharmacological or biochemical evidence to support the existence of a bona fide menthol binding site on sensory neurons, nor is it clear whether menthol and cold act through the same molecular entity to depolarize these cells.
Cold/menthol receptor gene and related genes have been reported in the literature under various different names. For example, McKemy et al., Nature 416:52-58 (2002) refer to this gene as CMR1 and suggest the role of this gene as a cold receptor and also suggest a possible general rule for TRP channels in thermosensation. Also, Peier et al. (Cell 108(5):705-715 (2002) and Science 296:2046-9 (2002)) refer to a TRP channel that they name to TRPM8, which is reported to be a distant relative of VR1, that is activated by cold temperatures and by a cooling agent, menthol. Additionally, Tsavaler et al. and others of Dendreon Corporation refer to a gene related to CMR1 by the names Trp-8 and SP 1-4, and teach that this gene is upregulated in prostate cancer and other malignancies.
The current invention is based on the discovery that the molecular site of menthol action is an excitatory ion channel expressed by small-diameter neurons in trigeminal and dorsal root ganglia. Remarkably, the cloned channel is also activated by cold (8 to 28° C.), demonstrating that menthol does, indeed, elicit a sensation of cool by serving as a chemical agonist of a thermally responsive receptor. This cold- and menthol-sensitive receptor (CMR1) exhibits the highest similarity to members of the so-called long TRP or TRPM channel subfamily, making it a close molecular cousin of the heat-activated channels, VR1 and VRL-1. Thus, TRP channels are the primary molecular tranducers of thermal stimuli and pain related to thermal stimuli within the mammalian somatosensory system.